The following co-pending patent applications are hereby incorporated by reference: U.S. patent application Ser. No. 07/626,355, filed Dec. 11, 1990; U.S. patent application Ser. No. 07/785,671 filed Oct. 31, 1991; U.S. patent application Ser. No. 07/817,206 filed Jan. 6, 1992; and pending U.S. patent application entitled An Improved Adaptive Leak HDTV Encoder.
In U.S. patent application Ser. No. 07/625,355 an encoding scheme is disclosed together with a corresponding decoder. The disclosed encoding and decoding is proposed for a terrestrial high definition television ("HDTV") environment in the United States, but the disclosed principles have much broader applicability.
To briefly describe the disclosed arrangement, the input signal to be encoded is, for example, a video signal that comprises a concatenation of signal segments that correspond to signals that make up an image frame. That signal is evaluated in a forward estimation portion of the encoder and various parameters are developed by that evaluation. Thereafter, with the aid of the developed parameters, the signal is encoded and thereby substantially compressed, buffered, modulated and finally transmitted to the decoder (e.g., an HDTV receiver). Some of the parameters developed in the forward estimation portion of the encoder are also transmitted to the decoder, including motion vectors and image mean signals. In accordance with the teaching of U.S. patent application Ser. No. 07/625,355, the signals transmitted by the encoding portion comprise scale factor signals, quantized vector signals and vector codebook identifier signals.
In carrying out the differential encoding process, the encoder must be aware of the signals that the target receiver has, in order for the encoder to take account of whatever encoding/decoding errors are introduced by the process and cannot be avoided. To that end, the encoder includes a frame buffer which is populated with signals which are derived from the encoded signals created by the encoder.
At the decoder (i.e., at the receiver) the received signals are decoded with the aid of a frame buffer which maintains the previously decoded signals. The frame buffer within the decoder corresponds to the aforementioned frame buffer within the encoder.
One problem with this approach, when strictly executed, is that errors introduced into the signal following the encoding process show up in the decoder's frame buffer and do not disappear. To ameliorate this potential problem, the disclosed encoder arrangement introduces a signal leak. That is, the encoder does not encode the difference between the current frame and a prediction of the current frame derived from the previous frame which is stored in the frame buffer. Rather, the signal which is encoded is the difference between the current frame and only a fraction of the prediction frame. In effect, a portion of the current frame is encoded, because it is not nullified by the prediction frame. That portion, called the "leak", is controlled in accordance with image characteristics and other parameters that relate to the encoder's operation. The actual control is realized by providing what effectively is a multiplication circuit responsive to the output of the frame buffer, which circuit multiplies its applied signals by a fraction. The leak signal is also transmitted to the decoder.
In pending U.S. patent application entitled An Improved Adaptive Leak HDTV Encoder it is also disclosed that the encoder includes an output buffer. In order to maintain a constant delay between the encoder's input signal and the decoder's output signal, it is important to know the level of fullness of the encoder's output buffer (e.g., in terms of the number of image frames stored therein). Accordingly, pending U.S. patent application entitled An Improved Adaptive Leak HDTV Encoder discloses an encoder that also transmits an output buffer fullness control signal.
The arrangement described above works well in that an injected perturbation, such as transmission noise entering the frame buffer, is removed within a number of image frames by virtue of the multiplication-by-a-fraction process that occurs at the output of the frame buffer. Still, such perturbations are not welcome, and whenever there is a priori information that such a perturbation is about to manifest itself, it would be beneficial to overcome it quickly.
The effects of perturbations are mitigated by reducing the time needed to converge the decoder's prediction signal to that of the encoder. This is accomplished by temporarily altering the leak factor when it is known that a perturbation is about to manifest itself and by, in some situations, altering the incoming encoded signal. In particular, when a decoder tuned to a particular encoder is directed to receive the signals of a different encoder (e.g., a channel change on an HDTV receiver), the incoming leak factor signal is set to 0 for one frame. During that frame, signals that are applied to the decoder are multiplied by a factor related to the incoming leak factor (as long as the incoming leak factor is not equal to 1).
In addition to the subject matter disclosed in the aforementioned patent applications, there exist two other schemata for developing a digital signal corresponding to motion video: MPEG-I and MPEG-II.
MPEG-I is an arrangement for encoding a sequence of images employing progressive scanning. The salient characteristic of this arrangement is that there are three different encoding approaches which may be applied to a video frame. One approach is to encode the frame directly. Such a frame is typically called an "I" frame, where "I" stands for "intra" coding to designate that the encoding relies solely on information within the frame. A second approach is to encode the frame based on a prediction derived from a prior "I" frame, or from a prior frame encoded by this second approach. This prediction is somewhat similar to the prediction described in the aforementioned HDTV applications. Such frames are typically called "P" frames. The third approach is to encode the frame based on a prediction from an adjacent "I" frame, an adjacent "P" frame, or both. Such frames are typically called "B" frames. An "I" frame essentially resets the decoder, since "I" frame information completely describes the video image independent of other frames. MPEG-I specifies that the distance between "I" frames is fixed. MPEG-I also specifies the distance between "P" frames.
The MPEG-I approach incorporates a particular hierarchical signalling syntax. At the top of hierarchy is a Sequence Layer. Subsequent layers in sequence are Group of Pictures Layer, Picture Layer, Slice Layer, Macroblock Layer, and Block Layer. The important aspect of the MPEG syntax that should be noted is that information in the Block Layer and the Macroblock Layer relates to specific size blocks of video that have been properly encoded (i.e., 16.times.16 pixel macroblocks).
The MPEG-II approach is designed to handle field-interlaced motion video. In MPEG-II, the two fields to be encoded can be combined to form a single frame or they can be concatenated. The prediction operation which is carried out in the field-interlaced signal can make use of a corresponding field in the previous frame or the second field of the same frame.
Both MPEG approaches require a decoder which is reasonably substantial--particularly with regard to size of the buffer needed by the decoder. For example, in accordance with the MPEG approaches described above, this buffer must contain the information of at least 2 1/2 frames of motion video. Since memory is expensive, the complexity of the present decoders may be disadvantageous is some circumstances.